The objective of this project is to define the roles played by thyroid hormones and prolactin in the etiology and biology of breast cancer. This goal will be met by studies of triiodothyronine, prolactin and estrogen receptors, and mitochondrial alpha-glycerophosphate dehydrogenase in experimental mammary tumors induced by dimethylbenz(a) anthracene and N-nitrosomethylurea, and human breast cancer tissues. Special attention will be paid to the effect of prior hyperprolactinemia on these proteins. Clinical studies are concerned with the effect of adjuvant chemohormonal breast cancer therapies on thyroid function and plasma prolactin levels.